Form Is Not Fully Constrained
Not all aspects of form have been defined by the tolerances that have been applied to the feature or feature of size. The form of a feature or feature of size may be constrained in one of the following ways:
A form geometric tolerances (flatness, straightness, circularity, or cylindricity) applied to the surface of the feature.
A non-form geometric tolerance applied to the surface of the feature (e.g., perpendicularity applied to a planar surface imposes an equal implied flatness control to that surface). However, note that the tangent plane modifier, Ⓣ, eliminates the form control that would otherwise be imposed by a geometric tolerance (e.g., perpendicularity applied to a planar surface).
A size dimension applied to a feature of size (i.e., Rule #1 states that form variation must be less than or equal to the size tolerance).
It is generally considered to be good practice to specify the limits of form for each surface of the model (either explicitly or implicitly).